Peroxidic compositions

ABSTRACT

Compositions for the polymer curing, comprising: 
     A) one or more organic peroxides and 
     B) trans,trans-2,6-dimethyl-2,4,6-octatriene

The present invention relates to peroxidic compositions usable for the elastomer and polyolefin curing in general, having an improved scorch resistance without negatively affecting the curing efficiency.

More specifically it relates to compositions having a longer scorch time as shown by the ts₅ and ts₁₀ values combined with a good curing density as shown by the MH values and having t₉₀ curing times of the same order.

It is well known that organic peroxides are used for the elastomer and polyolefin curing. However it is desirable to have available compositions with higher scorch times in order to increase the compound processability avoiding precuring phenomena.

In order to solve this technical problem organic peroxides having a longer half-time could be used, however this has the drawback to lengthen also the curing times (t₉₀), to the detriment, therefore, of the productivity, in order to maintain the same curing value (MH).

Adding additives to the peroxidic compositions to increase the scorch time is also known. See for example DE 2,553,145 and 2,553,094 wherein peroxides with different scorch times are mixed but the drawback is that the curing times become longer with the above mentioned disadvantages.

Also amine-based additives are known, but today they cannot be used any longer since they are considered toxicologically dangerous due to their cancerogenous effects.

There are also known hydroquinones, e.g. di-t-butyl or di-t-amyl; sulphur-based compounds, or antioxidants in general as scorch retardants in the peroxidic curing, in this way however the final properties of the cured product are poor since the MH value becomes worse.

In U.S. Pat. Nos. 5,292,791 and 5,245,084 a peroxidic composition having scorch retardant properties is described, which comprises:

(a) a peroxide

(b) an hydroquinone derivative

(c) a curing promoter.

A scorch resistant composition, stable upon storage, is also known from the EP 785,229 in the name of the Applicant wherein a powdered peroxide or in the form of grain or masterbatch is mixed at the time of use with a masterbatch comprising an inhibitor and a curing promoter. In the above compositions of the European Patent Application and of the U.S. Pat. Nos. '791 and '084 two additives are used. One mainly acts as an inhibitor (scorch resistant) but has a negative effect on the curing degree.

In order to maintain the curing degree a curing promoter is added.

From the EP 533,089 peroxidic compositions are known wherein a solid peroxide bis(alpha t-butyl-peroxy-isopropyl)-benzene, known as PEROXIMON® F, is mixed with bis(alpha t-amyl peroxy-isopropyl)-benzene, known as PEROXIMON® 180. Said compositions are liquid also at temperatures of 15° C. or lower and show a low volatility. The advantage of said compositions consisted in having available particularly desired liquid compositions in compounding application according to continous (not batch) processes or in peroxide additions carried out by direct absorption on the polymers. The drawback of these compositions is that even though they maintain good MH values, they are not scorch resistant.

The need was felt to have available compositions with improved scorch time without notably lower the curing density (MH) with comparable t₉₀ curing times.

The Applicant has unexpectedly and surprisingly found compositions for the polymer curing which give the above mentioned property combination comprising:

a) one or more organic peroxides selected from the following general formulae:

    (R.sup.1 --C(CH.sub.3).sub.2 --OO--C(CH.sub.3).sub.2).sub.n --R.sup.2(I)

wherein R¹ is an alkyl, aryl and aryl-alkyl-substituted group having from 1 to 9 carbon atoms, R² is selected from phenylene, ethylene, --C═C--, --C═C--, alkyl, aryl and aryl-alkyl-substituted group; said groups having from 1 to 9 carbon atoms; n is an integer equal to 1 or 2; ##STR1## wherein the two R³ substituents are independently alkyl, aryl, aryl-alkyl-substituted groups having from 1 to 9 carbon atoms, R⁴ and R⁵ are independently linear or branched, when possible, alkyl groups, having from 1 to 6 carbon atoms or --(CH₂)_(m) --C(O)OR⁶ wherein R⁶ is an alkyl group having from 1 to 4 carbon atoms; m is an integer from 1 to 3, or they jointly form a not substituted or substituted cyclohexane or cyclododecane ring with 1 up to 3 alkyl groups having from 1 to 4 carbon atoms; and

b) trans,trans-2,6-dimethyl-2,4,6octatriene having the formula ##STR2##

The preferred type a) components are: dicumylperoxide (PEROXIMON® (DC)), t-butyl-cumylperoxide (PEROXIMON® 801), bis(α-t-butylperoxyisopropyl)benzene (PEROXIMON® F), 2,5-di(t-butylperoxy)-2,5-dimethylhexane (Luperox® 231), 2,5-di(t-butylperoxy)-2,5-dimethylhexyne-3 (Luperox® 130), diterbutylperoxide (Luperox® DI), 1,1-diterbutylperoxy)-3,3,5-trimethylcyclohexane (Luperox® 101), n-butyl-4,4-di(terbutylperoxy)valerate (Luperox® 230), 1,1-di-terbutylperoxycyclohexane (Luperox® 331), isopropylcumylterbutylperoxide (PEROXIMON® DC 60), bis(α-teramylperoxyisopropyl)benzene (PEROXIMON® 180). All these peroxides are commercialized by Elf Atochem.

The component b) amount for one hundred parts by weight (100 phr) of elastomer or polyolefin to be cured is in the range 0.05-1.7, preferably 0.1-1.3.

The weight ratio between the component a) and the component b) is in the range 1:0.02 to 1:0.7, preferably 1:0.05 and 1:0.6.

It is also possible to prepare formulations (in inert fillers and/or predispersions in a polymer) containing the (a+b) mixture in an amount generally higher than or equal to 30% to be used as additives to be dispersed in the polymer to be cured. The preferred range for said formulations is from 30 to 70% of (a+b) mixture. Said formulations are well known and can be prepared according to EP 785,229 herein incorporated by reference.

The polymers which can be cured according to the present invention are ethylene-based polymers. More specifically polyethylene having average, low, high density, poly-butene-1, ethylene/vinyl-acetate copolymers, acrylic ester/ethylene copolymers, ethylene/propylene copolymers, ethylene/butene-1 copolymers, ethylene/4-methyl-pentene-1 copolymers and propylene/butene-1 copolymers; furthermore ethylene/propylene ethylene/propylene or ethylene/propylene diene monomer type elastomeric polymers or copolymers, butyl rubber, chlorinated polyethylene and propylene/butene-1 copolymer, can be mentioned. Also mixtures of two or more polymers can be used.

The final compound ready for curing (polymer+peroxide a)+additive b)+mineral and non mineral fillers, antioxidants, curing coagents, etc., see the above mentioned EP 785,229, is preferably used to produce manufactured articles extruded in a continous way and/or injection and/or compression molded.

Curing can be carried out by heat and can be directly performed in the molding in the case of compression or injection molding; in the case of continuous extrusion with the known methods, for example vapour, nitrogen, melted salt baths, autoclaves curing lines, etc.

The polymer characterization in terms of scorch resistance (ts₅ and ts₁₀), curing rate (t₉₀) and curing density (MH) of the cured product, is reported in Example 1.

The following examples are given for illustrative purposes and they are not limitative of the present invention.

EXAMPLE 1

A blend was prepared by mixing 100 g of low density polyethylene (produced by ELF Atochem, commercialized as LACQTENE® 1020 FN 24), under the form of fine powder, and 2.5 g of dicumylperoxide (PEROXIMON® DC) and 0.3 g of trans,trans-2,6-dimethyl-2,4,6-octatriene.

The blend was prepared in a V mixer at 50° C. for 60 minutes.

The resulting blend was characterized by using a rheometer (α-Technologies and commercialized as ODR 2000) and a Mooney viscometer for the scorch tests (MV 2000 α-Technologies).

MH and t₉₀ have been obtained by the ODR curve at 180° C. (oscillation arc3°, oscillation frequency 100 cycles/min.). The ts₅ and ts₁₀ scorch times have been obtained by the scorch curve carried out by the viscometer MV 2000 at 160° C.

The t₉₀ term means the necessary time to reach a curing equal to 90% respectively of the curing curve maximum. MH represents the maximum couple on the same curve.

By ts₅ and ts₁₀ scorch time, the necessary time to obtain a viscosity increase of 5 or 10 Mooney units, with respect to the minimum value, is meant.

The results are shown in Table 1.

EXAMPLE 1A (Comparative)

The procedure of Example 1 has been repeated without using the component b) trans,trans-2,6-dimethyl-2,4,6-octatriene.

The results are reported in Table 1.

By comparing Example 1 with Example 1A it is noticed that the scorch time is clearly higher with the invention compositions and that at the same time better MH values are obtained, with comparable t₉₀.

EXAMPLE 2

Example 1 has been repeated but using 1 g of component b).

The results are shown in Table 1.

The comparison with the results of Example 1 and 1A shows a substantial improvement of the scorch time and maintaining the MH value of Example 1.

                  TABLE 1                                                          ______________________________________                                               Peroxide                                                                   Ex. (comp a)) Comp. b) ts.sub.5 ts.sub.10 t.sub.90 MH                          No. (g) (g) (s) (s) (s) (lb · inch)                                 ______________________________________                                         1     DC 2.5   0.3       249  279   385  24.1                                    1A DC 2.5 0 192 218 377 21.7                                                   2 DC 2.5 1 338 384 406 23.8                                                  ______________________________________                                    

EXAMPLES 3-19

Example 1 has been repeated but by using the amount of the peroxide compound (DC) and component b) reported in Table 2.

The blend was characterized by the measurement of ts₅ at 145° C. with the Mooney viscosimeter of Example 1.

                  TABLE 2                                                          ______________________________________                                                  Peroxide                                                                 (DC) Comp. b) ts.sub.5                                                        Ex. No. (g) (g) (145° C.)                                             ______________________________________                                          3       2.5          0         467                                               4 2.5 0.3  660                                                                 5 2.5 1.0  897                                                                 6 2.5 1.5 1028                                                                 7 2.5 2.0 1060                                                                 8 2.0 0  548                                                                   9 2.0 0.3  778                                                                10 2.0 1.0 1025                                                                11 2.0 1.5 1327                                                                12 2.0 2.0 1448                                                                13 1.5 0  661                                                                  14 1.5 0.3 1012                                                                15 1.5 1.0 1380                                                                16 1.5 1.5 1737                                                                17 1.0 0  858                                                                  18 1.0 0.3 1473                                                                19 1.0 1.0 2200                                                              ______________________________________                                    

The data in table 2 show that the additive b) provides an improved scorch protection (ts₅) at 145° C. This is extremely relevant since a temperature around 145° C. is the processing temperature of the final compound to obtain manufactured articles by extrusion.

This allows the user of this technology to enhance the productivity and the final product stability during the extrusion process. 

What is claimed is:
 1. A composition suitable for polymer curing, the composition comprising:a) one or more organic peroxides selected from the following formulae:

    (R.sup.1 --C(CH.sub.3).sub.2 --OO--C(CH.sub.3).sub.2).sub.n --R.sup.2(I)

wherein R¹ is selected from the group consisting of an alkyl, aryl and aryl-alkyl-substituted group having up to 9 carbon atoms, R² is selected from the group consisting of phenylene, ethylene, ethenylene, alkyl, aryl and aryl-alkyl-substituted group having up to 9 carbon atoms and n is an integer equal to 1 or 2; ##STR3## wherein the two R³ substituents are independently selected from the group consisting of alkyl, aryl and aryl-alkyl-substituted group having up to 9 carbon atoms, R⁴ and R⁵ are independently linear or branched alkyl groups having from 1 to 6 carbon atoms or --(CH₂)_(m) --C(O)OR⁶ wherein R⁶ is an alkyl group having from 1 to 4 carbon atoms and m is an integer from 1 to 3, or R⁴ and R⁵ together from a cyclohexane or cyclododecane ring which is unsubstituted or substituted with up to three alkyl groups each independently having from 1 to 4 carbon atoms; and b) trans,trans-2,6-dimethyl-2,4,6octatriene having the formula ##STR4##
 2. The composition according to claim 1, wherein the component a) is selected from the group consisting of dicumylperoxide, t-butyl-cumylperoxide, bis(α-t-butylperoxyisopropyl)benzene, 2,5-di(t-butylperoxy)-2,5-dimethylhexane, 2,5-di(t-butylperoxy)-2,5-dimethylhexyne-3, diterbutylperoxide, 1,1-di(terbutylperoxy)-3,3,5-trimethylcyclohexane, n-butyl-4,4-di(terbutylperoxy)valerate, 1,1-di-terbutylperoxycyclohexane, isopropylcumylterbutylperoxide and bis(α-teramylperoxyisopropyl)benzene.
 3. The composition according to claim 1, wherein the component b) amount per one hundred parts by weight (100 phr) of polymer to be cured is in the range of 0.05-1.7.
 4. The composition according to claim 3, wherein the component b) amount per one hundred parts by weight (100 phr) of polymer to be cured is in the range of 0.1-1.3.
 5. The composition according to claim 1, wherein the weight ratio between the component a) and the component b) is in the range of 1:0.02 to 1:0.7.
 6. A method of curing an ethylene-based polymer, the method comprising heating a mixture of an ethylene-based polymer to be cured and the composition according to claim
 1. 7. The method according to claim 6, wherein the ethylene-based polymer to be cured is selected from the group consisting of polyethylene having an average, low or high density, an ethylene/vinyl-acetate copolymer an acrylic ester/ethylene copolymer, an ethylene/propylene copolymer, an ethylene/butene-1 copolymer, an ethylene/4-methyl-pentene-1 copolymer, ethylene/propylene or ethylene/propylene diene monomer elastomeric polymers or copolymers, butyl rubber, chlorinated polyethylene and propylene/butene-1 copolymer.
 8. The composition according to claim 1, further comprising a polymer to be cured and optionally mineral and non-mineral fillers, antioxidants and/or curing coagents. 